The invention relates to seals used in connection with oil and gas extraction wells and, more particularly, a packing seal for a reciprocating pump.
Hydraulic fracturing is used for removal of petroleum, natural gas, coal seam gas and other flowable substances from beneath the earth's surface. Highly pressurized fluid is forced into a wellbore to create new fractures in a rock layer. After creating the fracture it is desirable to maintain the fracture width since this increases extraction rates and recovery of fossil fuels. Thus, material such as sand, ceramic, or other particulate, which is known as proppant, is mixed with the fluid and injected into the fracture to keep it open. The abrasive nature of the particulates, however, wreaks havoc on the piston/cylinder assemblies of the high-pressure pumps used in “fracking.”
When the particulates are allowed between the walls of the bore/plunger assemblies, loss of pressure results. Keeping them from reaching between the bore and cylinder plunger, therefore, is essential for seal duration. Well service packing (WSP) is typically used to seal the gap and permit slidable engagement. The packing is a collection of ring-shaped seals contained in a bore, known in the art as a “stuffing box,” and arranged in order so as to incrementally ride against the wall of the plunger and seal it at the fluid end. The bore receives the reciprocating plunger making replacement of all of the seals, which often must be done in the field, more manageable and convenient.
At the time of this writing, frictional wear of the header ring is perhaps at its worst. This is because the proppant now preferred in frac jobs has become smaller and smaller in size. To make matters worse, the seal has to withstand a range of different fluid pHs, too. That is, the pressurized fluid may be cement (mildly acidic) instead of water (neutral), for example. The material from which the seals are made, therefore, must be matched with a set of desired physical properties. Prior seals are made from elastomeric composites, which can be abrasive to the stuffing box even if there is no proppant between the seal and box. The seals are subjected to extremely high pressures and a broad range of operating temperatures as well. Hence, wear and tear of the seals are constant concerns.
Besides problems caused by not properly cleaning the contacting surfaces when replacing seals in the field, sealing problems are exacerbated by the mechanics of the stuffing box. Packing is secured in the box mechanically and secured about the plunger with a gland nut. If the nut is too tight, the header ring, which may be formed from a compressible material, may be extruded back into the fluid end, and the seal will fail. Conversely, if the gland nut is too loose, the seal assembly will move back and forth in the stuffing box causing wear and eventual failure of the seal. “Hammering” and damage to the metal adapter also results when the nut is too loose. Assuming the gland nut is properly secured, still, it may back off due to vibration of the pump.
There, therefore, remains a need for an improved packing seal for a reciprocating pump. It would be advantageous if such a seal was a single, unitary seal to ease shipping and handling. Another need is for the seal to handle otherwise excessive axial force delivered to the stuffing box by way of the gland nut without deleterious effect to the soft seal portions of the packing. The present invention is directed toward meeting this need.